The variable flow multiple pass apparatus for heating liquids of the present invention represents an advance over the prior art devices for heating large quantities of liquids such as cooking oil for use in the preparation of food products. The heating of the fluid bath in the food preparation process is currently carried out by direct flame impingement on the bottom of the conveyor trough. It is well known that this method of direct heating results in several disadvantages. Localized application of heat cannot provide a consistent temperature throughout the oil bath;, causing "hot spots" which result in oil degradation. Another disadvantage associated with such devices and methods is thermal inefficiency which results in higher energy costs. The device of the present invention overcomes these and other disadvantages by providing a variable flow multiple pass apparatus which is contained and separated from a cooking conveyor. The device provides for uniform and consistent temperature throughout the process and does so with greatly increased thermal efficiency.